Nitrogen (N2) and Carbon Dioxide (CO2) foams have been used as hydraulic fracturing fluids for several decades to reduce water usage and minimize damage in water-sensitive reservoirs. These foam treatments require gases to be liquefied and transported to site. An alternative approach would be to use natural gas (NG) that is readily available from nearby wells, pipelines, and processing facilities as the internal, gaseous phase to create a NG-based foam. Hydraulic fracturing with NG foam is a relatively inexpensive option, makes use of an abundant and often wasted resource, and may even provide production benefits in certain reservoirs. As part of an ongoing development project sponsored by the Department of Energy (DOE), the surface process to create NG foam is being developed and the properties of NG foam are being explored. This paper presents recent results from a rigorous pilot-scale demonstration of NG foam over a range of operating scenarios relevant to surface and bottomhole conditions with a variety of base-fluid mixtures. The Pilot-scale Foam Test Facility (PFTF) used in these investigations is first described. The PFTF is capable of generating foamed fluids at pressures up to 7,500 psig and at temperatures in excess of 300°F. Then, results from several investigations aimed at proving NG foam at conditions relevant to the field are presented. NG foam was characterized using rheology measurements and flow visualization techniques. Experiments were performed to investigate the texture and stability of NG foam generated by two different mixing methods: one using a custom designed tee to match mixing velocities in the field where the gas phase is jetted into the aqueous stream, and another to ensure comprehensive mixing for laboratory analysis. Parametric studies were conducted to explore the effects of flow rate, foam quality, and temperature on the stability of NG foam. Moreover, different fluid preparations were used to investigate the effect of base fluid and additive concentrations on the stability of NG foams. Additional laboratory studies that investigated foam stability with produced water and multicomponent NG mixtures are also reported. The NG foams explored in these investigations exhibited typical, shear-thinning behavior observed in rheological studies of N2- and CO2-based foams. The measured viscosity and observed stability indicate that NG foams are well suited for fracturing applications. Like other foams, NG foam exhibits sensitivity to operating temperature characterized by a decrease in apparent viscosity as temperature increases. Rapid foam breakdown was observed at significantly elevated temperatures exceeding 290°F. In addition to fluid characterization, these investigations also yielded several key lessons that should be applied to future field demonstrations of NG foam.

Gas-liquid scrubbers rely on level control systems (generally consisting of a level indicator, a level controller, and a pneumatic control valve for liquid release) to maintain an appropriate liquid level within the vessel. Scrubbers are often upstream of turbomachinery and failures at the scrubber can cause liquid ingestion or downtime. In natural gas service, these control systems are subject to harsh environments due to the influx of liquid slugs, high-velocity gases, corrosive fluids, vibrations, and a chaotic gas-liquid interface. In these severe conditions, level control system failures are commonplace and lead to safety and environmental hazards, equipment damage, and lost production. A need exists to augment or replace the typical liquid level control system with an alternative solution that is cost-effective, robust, and can operate reliably in the harsh natural gas environment. A project investigated failures related to scrubber level control systems, identified improvements to these systems, developed a prototype level controller, and tested the prototype controller and a variety of commercially available controllers at various conditions that emulated certain field conditions. The results of these tests gave insight into what type of controller may be best suited to the tested conditions and what controller options should be pursued further.